This invention relates to automated testing of electronic devices, such as silicon chips, electrical circuits, and electrical systems, and, more particularly, to an improved electrical contact assembly for use in conjunction with such testing.
In order to successfully manufacture and market high quality and price-competitive electronic devices, for example, silicon chips, they must be tested to insure that they meet rigid performance specifications. In the past electronic devices were complicated, difficult and expensive to manufacture, and testing was generally done manually, a time consuming procedure that contributed to the high cost of such devices. However, in time, the design and manufacturing procedures were improved, prices declined and demand surged. To keep pace with the improved manufacturing operations, improved testing practices had to be developed, both to automate the testing and to reduce the time for testing. Now that some such electronic devices are being mass produced and sold by the millions, the testing procedures and practices have become very sophisticated, and the automated testing equipment has grown more complex and expensive. For example, the cost of test equipment for some mass produced electronic devices may range from $200,000 to $2,000,000.
A typical automated system for testing the performance of silicon chips may include, for example, (1) a performance board which has a large number of printed circuits on its upper surface and connectors on its lower side, (2) electronic pin cards that engage with the lower connectors, (3) an electrical contact assembly, which has a plurality of contact areas, that is positioned on top of the printed circuits of the performance board, (4) a device under test board (D.O.T.) that rests on top of the contact assembly, (5) a socket on top of the device under test board, and (6) an electronic device to be tested that is plugged into the socket. Such test systems are well known to those skilled in the art and will be referred to hereinafter in greater detail.
Electrical contact assemblies, which are one of the components of a system for testing electronic devices, normally have 24, 60, 84, 120, or more or less, contact areas, pins, probes or lines, generally arranged in a pattern. Electrical contact assemblies have been made with spring loaded contacts, compressible metallic conductive materials, compressible conductive rubber (elastomer) and other structures. However, all such prior assemblies have a number of deficiencies.
Present spring loaded contact assemblies are expensive because of the housings required to contain them and the need for short low resistance and low inductance connections. Furthermore, it is difficult to maintain the spring loaded contacts in positions that permit them to operate freely in both directions without undue interference. Compressive metallic conductive materials were widely used for contact assemblies before the development of conductive rubber assemblies that provide a more economical and dependable circuit. Compressible conductive rubber materials are now widely used for contact assemblies because of their moderate cost and the ability of most test operators to quickly observe when such assemblies are defective. Also, replacement of defective compressible conductive rubber assemblies requires little time. However, conductive rubber assemblies are subject to short circuiting between adjacent lines whenever slight misalignment of test assemblies occurs. In addition, the rubber material sometimes is damaged and reduced in size due to wear, abrasion and abuse. As a result of repetitive compression and release, as well as normal shop abuse, the use of conductive rubber contributes to problems in electronic test systems. The problems with conductive rubber are further compounded by errors created by the resistance of rubber when measuring low voltage drops.